Field of the Invention
This invention relates to an improved process for producing concentrated solutions of hypochlorous acid.
"Hypochlorous acid" has long been known as a bleaching agent and as a reagent in the Preparation of organic compounds such as chlorohydrins and chloramines. It has only been available commercially, however, in relatively dilute solutions i.e. 20% or less by weight of HOCl.
More recently, a process has been described in U.S. Pat. No. 4,146,578, issued Mar. 27, 1979 to J. P. Brennan et al in which hypochlorous acid is produced by spraying an aqueous alkali metal hydroxide in droplet form into gaseous chlorine. The reaction produces a gaseous mixture of chlorine, hypochlorous acid vapor and dichlorine monoxide, and solid particles of an alkali metal chloride. The gaseous mixture is scrubbed in water to produce a concentrated aqueous solution of hypochlorous acid. However, because of the dilution of the gaseous product with water, the hypochlorous acid solutions produced were less concentrated than desired, for example, for the production of its inorganic salts.
A further development in this process has been described in W. O. 90/05111, published May 17, 1990 by J. K. Melton et al. In this process, the gaseous mixture of hypochlorous acid, chlorine and dichlorine monoxide and water is condensed to produce a concentrated hypochlorous acid solution containing 35 to 60 percent by weight of HOCl. Following the condensation, a non-condensed gaseous mixture of chlorine and dichlorine monoxide is recovered from the condenser and after heating, the gaseous mixture is returned to the hypochlorous acid reactor. Melton et al employ molar ratios of chlorine to alkali metal hydroxide of at least 20:1. Use of these high concentrations of chlorine are effective in increasing the efficiency of the process and further in diluting the non-condensed gaseous mixture containing dichlorine monoxide recovered from the condenser. The non-condensed gaseous mixture was heated to high temperatures as it served as a primary source of heat for the reaction. However, the use of high concentrations of chlorine gas increases the material and operating costs of the process.
Further, commercial production of hypochlorous acids having concentrations above 50% by weight of HOCl by the processes of the prior art has not been accomplished. The production of hypochlorous acids having concentrations above 50% by weight of HOCl requires gaseous mixtures containing very high concentrations of dichlorine monoxide. The limited teachings of the prior art state that to prevent explosions, gaseous mixtures containing dichlorine monoxide should be maintained at concentrations of Cl.sub.2 O below 25% by volume and at temperatures below 120.degree. C. These conditions, however, seriously limit production capacity. By requiring large volumes of diluent gases and low reaction temperatures, increases in plant capacity require large increases in equipment size and add significant increases in capital, energy and operating costs.
Thus there is a need for a process for producing hypochlorous acid solutions having a concentration of greater than 50% by weight of HOCl which provides high concentrations of dichlorine monoxide while reducing volumes of diluent gases.